# Aircraft conceptual design for optimal environmental performance
**Henderson, Ryan P., Joaquim RRA Martins, and Ruben E. Perez., The Aeronautical Journal 116.1175 (2012): 1-22.**

:::info
Henderson et al. performed single-objective optimization of emissions including CO2 (fuel burn) and NOx by using the augmented Lagrangian particle swarm optimization (ALPSO) algorithm, and then used the multi-objective genetic algorithm to perform Multidisciplinary design optimization (MDO) for these objectives. Based on these environmental constraints, they output corresponding design indicators, compared to those of an aircraft with minimum direct operating cost, and finally came up with a design framework based on pyACDT (Python aircraft conceptual design toolbox). Their work(aircraft) had high direct operating cost due to its low cruise speed, and high fuel burn due to poor engine efficiency. At the same time, they believe that another possible direction of work is to design large aircraft for short ranges (LASR). Through investigation they found that more than 90% of all flights are less than 1,500nm. They think if a larger aircraft is designed for shorter ranges, the additional structural weight required for longer flights is removed, making the aircraft lighter, which in turn reduces fuel burn. According to their assumption, the LASR could reduce overall emissions by replacing the long range aircraft operating on short routes and by replacing two or more flights of smaller aircraft with a single flight.
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**Hi Lijing.
Good job! I have some questions.**
1) What is objective of this research?
2) What are their original contributions?
3) Why do they conduct this research?
4) What method did they use and why they choose the method?
5) What are the "these environmental constraints" in the third line?
6) What are the "design indicators" in the third line?
7) What is the definition of "direct operating cost"
8) Is pyACDT opensource? if yes, please find the link
9) Find flaws of this sentence "Through investigation they found that more than 90% of all flights are less than 1,500nm."
10) Why the additional structural weight required for longer fliught can be removed?
11) What are their assumption in "According to their assumption, the LASR could reduce overall emissions {by replacing} the long range aircraft operating on short routes and {by replacing} two or more flights of smaller aircraft with a single flight." ? I can understand the second {by replacing} A with B, but I can not really understand the first {by replacing}. By replacing the long range aircraft operating on short routes with what?

Answer:
## 1) What is objective of this research?
I think objective of this research are those output when optimized by minimum DOC, minimum mission fuel burn(CO2) and minimum LTO NOx emissions. Name of these objective can be seen in table 5. But for the objective function, the answer should be fuel burn and DOC. I am quite confusing about this but I know the difference between these two part of answer.

**DJKim - Sorry, I am not asking about the objective functions of the optimization, but aim of this research "https://dictionary.cambridge.org/dictionary/english/objective"
objective - something that you plan to do or achieve.
So, objective of the research is what do they want to achieve in this research. Please rewrite the answer accordingly.**

The objective of this paper is an aircraft design optimisation framework which is used to design aircraft that minimise specific environmental metrics.

## 2) What are their original contributions?
I think if we consider the original contribution part, it should be their single and multi-objective optimisations on multiple aircraft and they also investigated the tradeoffs between the various environmental performance metrics. In addition, a large aircraft optimised specifically for shorter ranges has also presented. They think this type of aircraft would be efficient on heavily used short-range routes.

**DJKim - First, "if we consider the original contribution part, it should be~" Think about this sentence again. Do you understand what is 'it' here? and does this flow make sense to you? (Sorry I am not angry or I don't want to be offensive. Maybe try to translate it to Chinese and think whether this make sense to you.)**

The original contribution mainly in two parts:
In this paper, single and multi-objective optimizations are performed on multiple aircraft to investigate the tradeoffs between the various environmental performance metrics.
In addition, a large aircraft optimised specifically for shorter ranges has also presented. {They think this type of aircraft would be efficient on heavily used short-range routes.}

**DJKim - {} this is not their contribution. It is a conculsion.**

LIU - Well received, and I agree.

## 3) Why do they conduct this research?
Consideration of the environmental impact of aircraft has become critical in commercial aviation. The continued growth of air traffic has caused increasing demands to reduce aircraft emissions, imposing new constraints on the design and development of future airplane concepts. In this big area, new structure and method need to be come up with. Also, I think the review of MDO also shows their interesting in this part.

**DJKim - What is "this" in "In this big area," and what is "big area"?
In "Also, I think the review of MDO also shows their interesting in this part", first, it should be "their interest in this part." or "shows that they are interested in this part."
Second, what is "this part".
Third, I don't think you can write "they conducted this research because they are interested in this part" in the technical paper.
Please rewrite the answer.**

Consideration of the environmental impact of aircraft has become critical in commercial aviation. The continued growth of air traffic has caused increasing demands to reduce aircraft emissions, imposing new constraints on the design and development of future airplane concepts. However, with current technology levels, the net result will still be an absolute increase in global green house gas emissions. These emissions will continue to affect the climate, particularly with the expected 5% growth in air transport. Thus, an aircraft design optimisation framework which is used to design aircraft that minimise specific environmental metrics.

**DJKim - why is "specific" needed here?**

LIU - I want to express that there can be other kinds of environmental metrics in different problem.**

**DJKim - OK, but first, "Thus, an aircraft design optimisation framework which is used to design aircraft that minimise specific environmental metrics." is grammatically wrong. No main verb is here.**
**Second, I feel the last sentence is closing general sentence because of "Thus". Usually, In this kind of sentence, you only refer what you have mentioned previously. However, you never mentioned any context about the optimization framework is only to minimize 'specific' environmental metrics. So I think 'specific' is unnecessary for the closing general sentence. If you want to use 'specific', add something like.... "Specifically, CO2 and Nox emissions are the major concerns in aircraft design." and then you say "Thus, they conduct the search on an aircraft design optimisation framework that minimise specific environmental metrics."**

Well noted. I will change this illustration in the my next version of review.

## 4) What method did they use and why they choose the method?
For single objective optimisations, they use ALPSO, a gradient-free population-based optimisation method for unconstrained problems, this is the same situation as the design prolem. NSGA-II are used to hadle the multi-objective optimisation. After I looked up the principle of this method, I think there are two reasons for their use: on the one hand, this is a very popular algorithm, which reduces the complexity of the non-inferior sorting genetic algorithm, and has fast running speed and the ability to solve sets. The advantage of good convergence is the performance benchmark of other multi-objective optimization algorithms. On the other hand, it is very suitable for this research topic. By finding the Pareto solution when multiple objectives are considered together, the optimal solution for the overall optimization of the aircraft is found.

**DJKim - If you start the sentence with "For single objective optimisations,", the readers expect "For multi objective optimisations," from the next sentence, not "for unconstrained problmes". Also, I think ALPSO is not for unconstrainted problems. Augmented Lagrangian is a method to solve constrained problem by making it to unconstrained form using Lagrandian multiplier.**
**What is the meaning of non-inferior sorting genetic algorithm?**
**I think you mixed up the benefit of choosing "ALPSO" and "NSGA-II". Could you separate them and describe them again?**

In this article they use two methods:
1.For single objective optimisations, they use ALPSO, a gradient-free population-based optimisation method for unconstrained problems, this is the same situation as the design prolem. 
2.For multi-objective optimisation, they use NSGA-II. After I looked up the principle of this method, I think there are two reasons for their use: on the one hand, {this is a very popular algorithm,} which reduces the complexity of the non-inferior sorting genetic algorithm, and has fast running speed and the ability to solve sets. The advantage of good convergence is the performance benchmark of other multi-objective optimization algorithms. On the other hand, {it is very suitable for this research topic.} By finding the Pareto solution when multiple objectives are considered together, the optimal solution for the overall optimization of the aircraft is found.

**DJKim - "This is a very popular algorithm" should not be a reason to be used in the research. So... if I write the sentence, I would write it as follows.**

**"on the one hand, the algorithm reduces the complexity of non-inferior sorting genetic algorithm BY SOMETHING. It also has fast running speed and the ability to solve sets."**

**"It is very suitable for this research topic" can not be a reason. I am asking why the method is suitable for this research and you are saying "it is very suitable for this research topic"**

**You can not find "the" or "an" optimal solution by finding Pareto solution. Pareto solution is a set of optimal solutions.**

LIU - Ok, I see! Let me think again about why these methods are used and why these methods are applicable to this research topic. This question is the most difficult for me to answer. I personally think there are two reasons: I am not familiar with the above two methods, and I lack the actual operation of the design optimization framework in this article. So when I answer, I feel that the question is a bit "abstract" to me.**

**DJKim - What do you mean by "the question is a bit "abstract" to me". Do you think the question is hard to understand and does not clearly state what you need to answer?**

LIU - This was my question before our last meeting, I have solved my confusing after the meeting last Tuesday.

## 5) What are the "these environmental constrains" in the third line?
About this word I think it should be the design output like what we can see in table five. After rereading I think it should named Design variables.

**DJKim - I am talking about pronoun "these". When you use pronoun, you have to mention what it is before you use pronoun. However, you did not list environmental constraints and you just use "these environmental constraints" so I am asking to list what are "these environmental constaints".**

Sorry I missed this question last time. What I want to express in my review is: the environment is the main factor considered in the optimal design of this article, so the author puts forward two emissions——CO2 and NOx. In my understanding, these two emissions exist as a prerequisite control condition, that is, the author will mainly consider them as "targets" when optimizing and designing the framework. But I am also confused, because after reading the article repeatedly, I found that the objective function of this article is DOC and fuel burn. Then I sorted out an idea and I don't know if it is correct: First, the emissions of CO2 and NOx are part of the fuel burn function (fuel-specificemission index (EI)). Subsequently, DOC is the second objective considered in design optimization. Based on the two objectives, use MDO (I don't quite understand how to run it) to get the optimization result, that is, the output of the designed aircraft optimization (similar to the values of parameters such as aspect ratio and the model diagram of the conceptual design of the specific aircraft).

## 6) What are the "design indicators" in the third line?
**DJKim - Please answer the question.**

In my review design indicators means the design output like what we can see in table five. After rereading I think this word should be renamed as design variables.

<font color="#f00">**DJKim - Good! I agree that it should be "design variables". One thing I want to point out is that add "," (comma) after "After rereading".**</font>

## 7) What is the definition of "direct operating cost"
Direct Operating Cost (DOC means expenditure that is directly related to flight operation, such as flight crew allowance, aircraft fuel and oil, lease rental or deprecation, aircraft maintenance, insurance premium,ground handling, navigational charges, landing and parking charges and in- flight catering service.

**DJKim - Where is ")" ?
in-flight, not in- flight
What's the mathematical expression of DOC?**

Reanswer:
Direct Operating Cost (DOC) means expenditure that is directly related to flight operation, such as flight crew allowance, aircraft fuel and oil, lease rental or deprecation, aircraft maintenance, insurance premium,ground handling, navigational charges, landing and parking charges and in-flight catering service.
Expression of DOC: CDOC = CDEP +CINT +CINS +CF +CM +CC +CFEE
This expression I found here (formula 14.1):
https://www.fzt.hawhamburg.de/pers/Scholz/HOOU/AircraftDesign_14_DOC.pdf

<font color="#f00">**DJKim - Even if you attach the file, you need to state what is CDEP, CINT, CINS, CF, CC, and CFEE. What are they?**</font>

## 8) Is pyACDT opensource? if yes, please find the link
Not an open source, I guess is because it designed by Advanced Aircraft Design Lab Royal Military College of Canada, I found its document in the link below.
https://www.researchgate.net/publication/268572300_pyACDT_An_Object-Oriented_Framework_for_Aircraft_Design_Modelling_and_Multidisciplinary_Optimization

**DJKim - Good! However, "I guess {is because it} designed by Advanced Aircraft Design Lab Royal Military College of Canada{,}" it should be {.} not {,}. Also, {is because it} should be "it is because pyACDT is". One more thing I would like to point out is cause and effect relationship. "I guess is because it designed by Advanced Aircraft Design Lab Royal Military College of Canada," -- This sentence is not a reason for not being an open source. If you say "It is because pyACDT is developed for military purpose, which is exclusive to public use." It makes sense.**

LIU - Well noted! I will pay more attention to my expression next time, and I agree with your point that military should be the keyword to explain why pyACDT cannot be public use.

**DJKim - Good!**

## 9) Find flaws of this sentence "Through investigation they found that more than 90% of all flights are less than 1,500nm."
First of all, this sentence does not explain what 1500NM refers to. It may be the length of the aircraft or the length of the flight. This is directly aimed at the possible contradiction caused by this sentence. Second, this statement is inaccurate, because they did not investigate in the article but the data they obtained from the table.

**DJKim - Good! Then how will you revise it?**

According to the market analysis of Kenway et al, the author found that the flight distance of more than 90% of the flights are less than 1,500 nautical miles.

**DJKim - Very good!**

## 10) Why the additional structural weight required for longer flight can be removed?
Shorter distance means less necessary oil storage space and single oil storage volume. The structure weight is the empty weight (personal opinion), the calculation of this part of the weight is included in the take-off gross weight (W0), so when Wf decreases, both We and W0 will decrease without changing the passenger weight (based on the payload).

**DJKim - I think you would better use "fuel" instead of "oil"
What are We and Wf? You didn't define them.
What does "single oil storage volume" mean and explain why shorter distance means single oil storage volume?**

Shorter distance means less necessary fuel storage space and single fuel storage volume. The structure weight is the empty weight (personal opinion), the calculation of this part of the weight is included in the take-off gross weight (W0), so when Wf (fuel weight) decreases, both We (empty weight) and W0 will decrease without changing the passenger weight (based on the payload).

**DJKim - Actually it doesn't make sense to me. Empty weight can not be changed and it doesn't have any relation with fuel wight. How would you explain?**

I agree with your idea, I searched the concept in the book, there is indeed a problem with my idea, empty weight is defined in advance. Then my other explanation is that due to the shorter flight distance, the relevant structural design to support long-distance flights can be removed. I'm still thinking about what exactly can be removed .

<font color="#f00">**DJKim- OK**</font>

## 11) What are their assumption in "According to their assumption, the LASR could reduce overall emissions {by replacing} the long range aircraft operating on short routes and {by replacing} two or more flights of smaller aircraft with a single flight." ? I can understand the second {by replacing} A with B, but I can not really understand the first {by replacing}. By replacing the long range aircraft operating on short routes with what?
I am using the original text for this sentence. I think what it means is that first they want to replace long-distance operations with short-distance operations, and the second replacing is the specific approach (replacing one long-distance operation with multiple short-distance operations). From the perspective of distance, I think the conservation is satisfied.

**DJKim - Please don't copy the setence from the paper. Rephrase them based on your understanding. The followings are my tips for rephrasing the sentences.**
**1) Change a "clause" to "phrase" or vice versa.
2) Change direct/quated speech to indirect/reported speech or vice versa.
3) Replace active sentence to passive sentence or vice versa.
4) Use synonym (I usually check it from https://www.thesaurus.com/)
5) Change part of speech(= word class)**

Rewrite:
According to their assumption, the LASR may lower overall emissions by substituting long-range aircraft for shorter-distance flights. In this way, it is possible to use one aircraft instead of two or more.

**DJKim - Don't use abbreviation without stating it before you use. "Large Aircraft for Short Ranges (LASR)"**

**DJKim- You still didn't give me the answer for my first question. What are their assumption in "According to their assumption"? I think you never mentioned about this assumption. So it should be something like... "According to their {SOMETHING} assumption"**

**DJKim - If I wrote this sentence, I would write like this. "According to their SOMETHING assumption, their LASR can reduce overall emission by replacing small aircraft with many operations on short routes with large aircraft with lesser number of flights."**

Rewrite:
According to their assumption that large aircraft for short ranges (LASR) can reduce the environmental impact of aircraft for a fixed Mach number, range, and technology level, the LASR may lower overall emissions by substituting long-range aircraft for shorter-distance flights. In this way, it is possible to use one aircraft instead of two or more.

:::info
In order to reduce environmental impact of aircraft, the aircraft design optimization ~~structure~~ **framework** ~~needs to reduce~~ **should be able to consider (minimize)** aircraft emissions. Henderson et al. ~~used~~ **considered** CO2 and NOx as emissions, and performed single objective optimization for ~~objective functions~~, fuel burn ~~(a surrogate~~ for CO2 emissions~~)~~ and landing-takeoff cycle **for** ~~(~~NOx emission**s**~~)~~. ~~Aircraft optimized~~ **Optimization** for minimum direct operating cost (DOC) ~~are~~ **is** also ~~included~~ **performed** for comparison purposes. In single objective optimization**,** they use**d** an augmented Lagrangian particle swarm optimizer to minimize the corresponding environmental metrics. After that, the author used a parallel multi-objective genetic algorithm to perform multi-objective optimization (for the narrow-body aircraft) on two objective functions (the DOC and the mission fuel burn). By analyzing the Pareto front, they found that the intermediate solutions have lower wing sweep angles and larger wing spans than the minimum cost aircraft. In addition, the cruise Mach numbers decrease from the high value for the minimum cost aircraft towards a much lower value for the minimum fuel burn aircraft. Finally, the author propose~~s~~**d** a new concept - large aircraft for short ranges (LASR). According to the market analysis of Kenway et al, the author found that the flight distance of more than 90% of the flights are less than 1,500 nautical miles. Thus, the author came up with an assumption that large aircraft for short ranges (LASR) can reduce the environmental impact of aircraft for a fixed Mach number, range, and technology level, the LASR may lower overall emissions by substituting long-range aircraft for shorter-distance flights. In this way, ~~it is possible to use~~ one aircraft ~~instead of~~ **can substitute** two or more.
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